1. The use of paid shills to create the impression of a popular
movement, through means like letters to newspapers from soi-disant
‘concerned citizens’, paid opinion pieces, and the formation of
grass-roots lobbying groups that are actually funded by a PR group
(AstroTurf is fake grass; hence the term). See also sock
puppet, tentacle.

2. What an individual posting to a public forum under an assumed
name is said to be doing.

This term became common among hackers after it came to light in early
1998 that Microsoft had attempted to use such tactics to forestall the
U.S. Department of Justice's antitrust action against the company. The
maneuver backfired horribly, angering a number of state attorneys-general
enough to induce them to go public with plans to join the Federal suit. It
also set anybody defending Microsoft on the net for the accusation
“You're just astroturfing!”.